1. Field of the Invention
The present invention relates to an ozone generator and an ozone generating method, and particularly to an ozone generator which includes a high voltage electrode and a low voltage electrode, causes discharge by the application of an AC voltage between them, and generates an ozone gas, and an ozone generating method.
2. Description of the Related Art
In the related art, various techniques as described below have been developed.
JP-B-6-21010 discloses an ozone generator in which a raw material gas is supplied from a first raw material supply system for supplying a specified flow rate of oxygen from an oxide cylinder with a purity of 99.995% or more, and from a second raw material supply system for supplying a specified flow rate of second raw material gas (nitrogen, helium, argon, or carbon dioxide) with a purity of 99.99% or more, a high AC voltage is applied between electrodes to cause silent discharge (dielectric barrier discharge) through a dielectric between the electrodes, and the raw material gas is transformed into an ozone gas. The publication discloses that although the cause of a time-varying reduction phenomenon of ozone concentration is not clear, the time-varying reduction phenomenon exists in the ozone gas once generated by the ozone generator under high purity oxygen, and as means for suppressing the time-varying reduction, it is effective to add a nitrogen gas or the like to the high purity oxygen.
Japanese Patent No. 2641956 discloses that a mixture ratio of an oxygen gas as a raw material gas of an ozonizer to a nitrogen gas is set in a range of 1:0.0002 (200 ppm) to 0.0033 (3300 ppm). Besides, FIG. 2 of Japanese Patent No. 2641956 shows a characteristic of the quantity of addition of nitrogen gas and the concentration of ozone obtained by the ozonizer, and as the quantity of addition of nitrogen at which sufficient ozone concentration (about 100 g/m3 or more) is obtained, the mixture ratio is set to 1:0.0002. In order to suppress the quantity of generation of nitrogen oxide as a reaction poisonous substance from the ozonizer to be small, the mixture ratio is set to 1:0.0033 or less. The publication discloses that when the oxygen raw material gas in which the quantity of addition of nitrogen is 100 ppm or less is used, the ozone concentration of 20 g/m3 (9333 ppm) is merely obtained, which is ⅙ or less of the ozone concentration of 120 g/m3 (56000 ppm) at the time when the quantity of addition of nitrogen is 3300 ppm. Besides, in the specification, it is disclosed that although an argon gas instead of the nitrogen gas is added to the high purity oxygen, the ozone concentration of about 20 g/m3 (9333 ppm) is merely obtained independent of an argon mixture ratio, and the argon gas does not have an effect to raise the ozone concentration.
Besides, JP-A-11-21110 discloses an ozone generator in which a TiO2 film is formed on a discharge surface of a dielectric. Instead of the addition of high purity nitrogen gas, the discharge surface of the dielectric in the generator is coated with titanium oxide having a metal element ratio of 10 wt % or more. It is disclosed that when the material gas is supplied and the ozone gas is generated in this generator, the time-varying reduction of ozone concentration can be prevented by the photocatalytic action of TiO2. In the case where a nitrogen gas is added to high purity oxygen to stably generate ozone, nitrogen oxide (NOx) as a reaction poisonous substance is generated as a by-product of the ozone gas by silent discharge. However, it is pointed out that as means for preventing the generation thereof, the coating of the titanium oxide is effective.
Further, Japanese Patent No. 2587860 proposes that in an ozonizer which can obtain a maximum ozone concentration of 180 g/m13 the quantity of addition of nitrogen is made 0.01% to 0.5% in order to suppress the time-varying reduction of ozone concentration.
In the related art, with respect to the mechanism of generating the ozone gas by silent discharge, it is said that the ozone gas is generated by following reaction equations.e+O22O+e(dissociation of oxygen)  R1O+O2+MO3+M(ozone generation based on triple collision by oxygen atom and oxygen material)  R2e+O3O+O2+e(electron collision decomposition)  R3O3+heat TO+O2(heat decomposition)  R4O3+NO2+N1(decomposition of ozone by impurity N)  R5
Incidentally, N1 denotes an impurity different from N.
The generation of the ozone gas is such that the oxygen molecule is dissociated to the oxygen atoms in R1, and the ozone is generated based on the triple collision by the oxygen atom and the oxygen material in R2.
As the decomposition of the generated ozone, the electron collision decomposition of R3, the heat decomposition of R4, the decomposition of ozone by the impurity of R5, or the like is conceivable.
As the ozone gas which can be extracted from the generator, the ozone gas is obtained according to the balance state of the reaction equations of R1 to R5. That is, the ozone gas can be extracted by a following equation.extractable ozone=(R1*R2)−(R3+R4+R5+ . . . ).Besides, in the related art, in the case of the high purity oxygen, with respect to the ozone generated by the ozone generation mechanism, since the ozone concentration is reduced with the passage of time during the operation, the nitrogen gas is added to the raw material gas, or TiO2 as the photocatalyst is applied to the discharge electrode surface, so that a following reaction occurs, and the time-varying reduction of the ozone concentration is prevented.O3*+N2O3 O3*+TiO2O3 
JP-B-6-21010, Japanese Patent No. 2641956, and JP-A-11-21110 are for stably obtaining the ozone concentration at a relatively low ozone concentration of about 120 g/m3.
Besides, Japanese Patent No. 2587860 discloses to obtain an ozone concentration of about 180 g/m3 or less.
Incidentally, in the respective related art, different phenomena as described below are described.
Although JP-B-6-21010 discloses that a gas of helium, argon, or carbon dioxide is also effective as a gas other than the nitrogen gas, Japanese Patent No. 2641956 discloses that in the case of the high purity oxygen, the argon gas is not effective.
Although JP-B-6-21010 discloses that the quantity of addition of the second raw material gas is made 10000 ppm to 100000 ppm, Japanese Patent No. 2641956 discloses 200 ppm to 3300 ppm which is different from the former.
JP-B-6-21010 discloses that in the high purity oxygen, the concentration is reduced by the operation for about one hour, while JP-A-11-21110 discloses the concentration reduction after the operation for about 7 hours, which is different from the former.
As described above, in the related art in which the nitrogen gas or the like is added to the ozone generated by the apparatus in order to suppress the time-varying reduction of the ozone concentration, the results and effects vary according to the conditions, and although experimental confirmation was made for JP-B-6-21010, Japanese Patent No. 2641956 and JP-A-11-21110, JP-B-6-21010 and JP-A-11-21110 could not be substantiated, and it turned out that addition of a separate noble gas (helium, neon, argon, xenon, etc.) other than nitrogen was ineffective.
Both JP-B-6-21010 and Japanese Patent No. 2587860 disclose that the reduction of the ozone concentration is the time-varying reduction, however, it is disclosed that when the concentration is once reduced, it does not return to the original ozone concentration. From the recitation that the concentration does not return to the original ozone concentration, it can not be judged that the concentration reduction is the time-varying reduction, and the role of the addition of nitrogen is not clear.
Further, it turned out that when the nitrogen was added at an additive rate of approximately 0.15% (1500 ppm) or more, in addition to the ozone gas, a large quantity of NOx by-product gas such as N2O5 or N2O was generated by the silent discharge.N2O5+H2O2HNO3 OH+NO2+MHNO3+M
Besides, when a large quantity of NOx by-product is generated, a nitric acid (HNO3) cluster (vapor) is generated by the reaction of the NOx gas component and moisture contained in the raw material gas, and the ozonized gas is extracted in such a state that a trace quantity of NOx gas and nitric acid cluster, together with oxygen and ozone gas, are mixed. When the quantity of the trace quantity of nitric acid cluster contained is several hundred ppm or more, there are problems that rust of chromium oxide or the like is deposited by nitric acid on the inner surface of a stainless pipe as an ozone gas outlet pipe, a metal impurity is mixed into a clean ozone gas, the metal impurity as a reaction gas for a semiconductor manufacturing apparatus has a bad influence on the manufacture of a semiconductor, and the trace quantity of the generated nitric acid cluster has a bad influence as a reaction poisonous substance on “an etching process of a silicon oxide film by ozone” or “ozone water washing of a wafer or the like” of a semiconductor manufacturing apparatus.
In the ozone apparatus of the related art, the concentration of the extracted ozone is low, and in order to extract ozone with a high concentration of 200 g/m3 or more, there is only a method of increasing the nitrogen additive rate or a method of decreasing the gas flow rate. In the method of increasing the nitrogen additive rate, as described above, there is a problem that the by-product gas of NOx is increased.
Besides, when the gas flow rate is decreased, there are problems that the quantity of ozone generation is extremely lowered, and production efficiency on the side of using the ozone becomes worse.
Further, in the newest “etching apparatus of an oxide film by ozone” or “ozone water washing of a wafer or the like”, a high ozone concentration of 200 g/m3 or more is needed, and with respect to the quantity of ozone generation, there is a request for an ozone apparatus having an ozone capacity of several tens g/h or more on an economically viable basis in production on the user side, and further, in a semiconductor manufacturing apparatus, an apparatus producing less reaction poisonous material such as nitric acid has been needed.
Besides, although a trace quantity, about 1%, of N2 gas is added in order to increase the generation efficiency of an ozone gas, the N2 gas is transformed into NOx or nitric acid cluster by discharge in the generator.
Thus, there are problems that in the discharge space, as the gas flow velocity becomes low, or the injected discharge power becomes high, the quantity of addition of nitrogen is decreased at the downstream part of the discharge space, a large quantity of NOx and nitric acid cluster are generated, the ozone generation efficiency is lowered, and the concentration of the extracted ozone is reduced.